This invention relates to a switching power supply for converting an alternating current line voltage to a variable and regulated direct current output voltage. The invention finds particular application in providing a direct current (DC) power supply from an alternating current (AC) source, wherein voltage isolation is desired between the AC source and the DC power.
Typical prior art power supplies utilize transformer coupling in one form or another. The use of the transformer provides the ability to convert an input signal from a first input AC voltage to a second, usually lower, output AC voltage, and also provides a means for obtaining electrical isolation between the output and input lines. Among the disadvantages in utilizing a transformer in such circuits is that the transformer typically dissipates about 20%-50% of the power applied to it, and the heat caused by this power dissipation must be conveyed from the circuit. Transformers are typically the largest and heaviest, and most costly, component of a switching power supply.
In a typical "chopper" switching power supply an input AC voltage is applied to a transformer and is stepped down to a lower AC output voltage. The AC output voltage is rectified and filtered to provide a DC voltage which may then be converted to a high frequency square wave via semiconductor switches and a controller circuit. The duty cycle of this square wave signal is controlled to provide a measured output DC power level, and power feedback is utilized to regulate the controller. The final DC voltage is formed from the square wave voltage signal by rectifiers and/or filters.
Another typical prior art power supply is an inverter switching power supply, wherein an input AC line voltage is rectified and filtered to form a DC voltage, and the DC voltage is then converted to a high frequency square wave which is coupled to a high frequency transformer. The transformer output signals are then rectified and filtered to provide a regulated DC output voltage, and a form of voltage or current feedback is used to control the output. The transformer utilized in an inverter switching power supply is typically of smaller size and lower power capability than a transformer in a chopper power supply, but the transformer is still a relatively large and costly component in the power supply circuit, and has a low efficiency.
There is an advantage in providing a DC power supply wherein voltage isolation may be maintained between an AC power source input and a regulated DC output voltage. There is a further advantage in utilizing some form of feedback to control and regulate DC output voltage or current, and it would be useful to provide a power supply having these characteristics without requiring the use of a transformer.